Conventional, a thermal-type air flow measuring device for measuring an air flow rate by use of heat transfer between the device and air is widely known. The air flow measuring device is disposed in an intake passage leading to an internal combustion engine to be used for measuring the flow rate of intake air suctioned into the engine (flow rate of intake air may be hereinafter referred to as an intake air amount).
More specifically, this air flow measuring device takes in a part of intake air flowing through the intake passage and generates a signal in accordance with the intake air amount. The air flow measuring device includes a housing that defines a bypass flow passage through which the taken-in intake air flows, and a flow sensor that produces the signal in accordance with the intake air amount as a result of heat transfer between the sensor and the intake air taken into the bypass flow passage. The air flow measuring device reduces the influence of turbulence of a flow of intake it in the intake passage by arranging a sensing part of the flow sensor in the bypass flow passage instead of disposing the sensing part directly in the intake passage, through which intake air passes, so as to output a measurement value with few variations.
In controlling the combustion of the engine, a humidity of intake air influences a propagation time of a flame. Accordingly, in controlling the combustion of the engine with gasoline as a fuel, for example, ignition timing is delayed or advanced in accordance with the humidity of intake air to optimize combustion efficiency. For this reason, an air flow measuring device which incorporates therein a humidity sensor in order to measure the humidity of intake air necessary for the combustion control of the engine is known (see, for example, JP-A-2010-043883 (corresponding to US2010/0031737A1), JP-A-H07-229776, and JP-A-H10-197305).
A housing of the air flow measuring device described in JP-A-2010-043883 includes a sub-air passage (corresponding to the bypass flow passage) that bypasses a main air passage (corresponding to the intake passage). Furthermore, the housing includes a second sub-air passage that bypasses the sub-air passage. A heating resistor and air temperature detecting part (these components correspond to the sensing part of the flow sensor) are disposed in the sub-air passage, and a humidity detecting part is set inside the second sub-air passage.
The air flow measuring device described in JP-A-H07-229778 includes a small passage (corresponding to the bypass flow passage) that bypasses main air passage (corresponding to the intake passage). A heating resistor and temperature-sensitive resistor (these components correspond to the sensing part of the flow sensor) are arranged in the small passage. Furthermore, a humidity detecting unit is disposed in the small passage on a downstream side of the heating resistor and temperature-sensitive resistor.
The air flow measuring device described in JP-A-H10-197305 includes a sub-passage that takes in a part of air flowing through an intake passage (corresponding to the bypass flow passage). A heating resistor and temperature compensation resistor (these components correspond to the sensing part of the flow sensor), and a semiconductor substrate, on which a humidity sensor is formed, are arranged in the sub-passage. Moreover, the humidity sensor is surrounded with a protector on the semiconductor substrate, and the air of the sub-passage is taken into the region enclosed by the protector through a vent hole.
However, in the air flow measuring device described in JP-A-2010-043883, JP-A-H07-229778, and JP-A-H10-197305, all the humidity sensors are disposed in the sub-passage which bypasses the intake passage and so forth. Therefore, thermal insulation from, for example, a resin material, from which the sub-passage is made, is insufficient, and a precision of the sensor may be reduced due to a heat exchange with the resin material. In addition, a flow of air in the sub-passage and so forth is easily influenced by a rail surface resistance as compared with inside the intake passage, and a speed of the flew thereby becomes slow. Consequently, the responsivity of the sensor may be also low.